Conventionally, hybrid construction machines have been proposed whose drive mechanisms are partially motorized. Such construction machines have a hydraulic pump for hydraulically driving working elements such as a boom, an arm, and a bucket, and have a motor generator connected via a speed-increasing machine to an engine for driving this hydraulic pump, so as to assist the driving of the engine with the motor generator and to charge a battery with the power produced by power generation.
Further, such construction machines have an electric motor in addition to a hydraulic motor as a power source for a rotation mechanism for rotating an upper rotating body. Such construction machines assist the driving of the hydraulic motor with the electric motor at the time of accelerating the rotation mechanism, and perform a regenerative operation with the electric motor to charge the battery with generated power at the time of decelerating the rotation mechanism. (See, for example, Patent Document 1.)
[Patent Document 1] Japanese Laid-Open Patent Application No. 10-103112